A modern motor vehicle typically has three tubes that run from the fuel tank, normally mounted under the rear of the vehicle, to the engine, normally in the front of the vehicle, that is a fuel line, a return line, and a vent line. Each of these tubes conducts fuel, which may be gasoline, diesel, or gas, or vapors of this fuel between the tank and various parts of the engine, the fuel and return lines to the fuel pump and the vent line to the intake manifold. As a rule the tubes are quite small, at most 15 mm in outside diameter.
In view of the highly flammable contents of such tubes, they must meet high crash standards. In a serious accident the tubes should not rupture and the fittings that attach the ends of the tubes to the tank and engine should not pull out. Any failure could release fuel into an accident site where it could pose an enormous danger.
Since the path such tubes must follow is invariably somewhat circuitous, in the oldest systems each tube was formed by a plurality of rigid and normally straight sections joined by flexible rubber hoses. The hoses were situated where the tube had to curve to get around something and also allowed the parts of the tube to move relative to each other, as in a crash, without leakage. The rigid parts, which were made of metal or plastic, could be shaped somewhat if necessary and were normally clamped to the vehicle body.
It has also been suggested to make the tubes wholly of plastic. The problem with this solution is that the fuel can diffuse into and even through many standard resins, so that it is necessary to coextrude an inner layer in the tube of a blocking resin, that is one that the fuel cannot diffuse through. Such a blocking resin is invariably fairly rigid so that the overall tube can bend very little. Thus it is necessary to preshape the tube, normally with heat, for the particular vehicle. Such a tube has little give, however, so that it cannot normally meet modern-day crash standards.